It is generally known that a plastic film introduced into a sound field attenuates said field to a greater or lesser degree. The thicker the film in relation to the wave length of the sound wave, the greater the attenuation.
A sound wave of a frequency of e.g. 40 kHz has a wave length of approx. 9 mm in air. When a plastic film of a thickness of e.g. 0.50-0.10 mm is introduced, the sound field is typically attenuated with 6-20 dB. If a reliable detection of a plastic film in a sound field is required and the production tolerances of the transmitter and the receiver are e.g. .+-.3 dB, an attenuation of only 6 dB will cause problems.